This invention pertains to display systems and more particularly to those systems using display devices which must utilize the same data several times during the display operation.
The most common of such display devices are cathode ray tube (CTR) displays which need periodic refreshing. However, not only CRT displays but also ink jet and laser displays wherein the characters are formed by matrices of dots require receiving the data several times to form a line of characters. A typical such display device is shown in U.S. Pat. No. 3,754,229 wherein a data source feeds the codes (code combinations of bits) for a line of characters to an end-around shift register acting as a line memory, i.e., a memory storing the character codes associated with one line of display. The codes are fed from the shift register to a translator where they are converted to rows of dot signals which are fed to a CRT. During each circulation a different row of dot signals is generated for each character. Thus if the dot matrix is an m.times.n matrix, then there are n circulations of the data. Such a display system has limited flexibility.
If the number of characters to be displayed per line is a variable due to multiple pitch capability there may be some more cells in the line memory than are needed. For example, a 10 pitch display shows 72 characters and the proportional width display up to 123. If this memory were a shift register, then during the read out mode 51 shifts could be required in the horizontal retrace interval of the CRT to return to the head of the line. During, the write mode, i.e., filling the buffer, the device which transfers from the character source to the line memory need only transfer characters up to an End of Line Code (or equivalent) in the allotted time. The speed of transfer is dependent on available character source accesses because of the processing of flags and the discarding of Non-Display characters which may be embedded in the character streams. In using a shift register the constraints on loading time become severe as the entire register must be shifted in one screen line time (horizontal scan line time) to reposition to the first characters.